Emission computed tomography (ECT) includes, for example, a positron emission tomography (PET) system and a single photon emission computed tomography (SPECT) system. The PET system has been applied widely in imaging, especially for medical diagnosis and/or treatment of tumors, heart diseases, brain diseases, etc. A PET system may be integrated with one or more other imaging systems to form a multi-modality system. Exemplary multi-modality systems may include a positron emission tomography-computed tomography (PET-CT) system, a positron emission tomography-magnetic resonance (PET-MR) system, etc. In a PET scan, attenuation to various extents may occur when γ-rays pass through different tissues of a subject because the attenuation degrees of different tissues to γ-rays are different, causing distortion of a PET image and/or PET data. To reconstruct a PET image and/or PET data, the attenuation may be corrected. An attenuation map may be generated in the process of attenuation correction.
In general, a method based on a CT image may be used to correct the attenuation of a PET image and/or PET data. However, the method based on a CT image (or referred to as a CT-based method) may cause a problem, such as low resolution of soft tissue imaging. A method based on a magnetic resonance (MR) image (or referred to as a CT-based method) may be used to correct the attenuation of a PET image and/or PET data. Compared to a CT-based method, the MR-based method may be provide a high sensitivity and/or high accuracy. However, the MR-based method may have one or more shortcomings in view of at least the followings. There is no mapping relation between the attenuation coefficient distribution of an MR image and the attenuation coefficient distribution of a corresponding PET image. Moreover, there may exist an edge truncation artifact in MR imaging.
Thus, there exists a need in the field to provide a method and system for attenuation correction that may address these and other technical challenges.